1. Field of the Invention
The present invention relates to wellbore completion. More particularly, the invention relates to methods for drilling with casing and landing a casing mandrel in a subsea wellhead.
2. Description of the Related Art
In a conventional completion operation, a wellbore is formed in several phases. In a first phase, the wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string while simultaneously circulating drilling mud into the wellbore. The drilling mud is circulated downhole to carry rock chips to the surface and to cool and clean the bit. After drilling a predetermined depth, the drill string and bit are removed.
In a next phase, the wellbore is lined with a string of steel pipe called casing. The casing is inserted into the newly formed wellbore to provide support to the wellbore and facilitate the isolation of certain areas of the wellbore adjacent to hydrocarbon bearing formations. Generally, a casing shoe is attached to the bottom of the casing string to facilitate the passage of cement that will fill an annular area defined between the casing and the wellbore.
A recent trend in well completion has been the advent of one-pass drilling, otherwise known as “drilling with casing”. It has been discovered that drilling with casing is a time effective method of forming a wellbore where a drill bit is attached to the same string of tubulars that will line the wellbore. In other words, rather than run a drill bit on smaller diameter drill string, the bit or drillshoe is run at the end of larger diameter tubing or casing that will remain in the wellbore and be cemented therein. The advantages of drilling with casing are obvious. Because the same string of tubulars transports the bit as it lines the wellbore, no separate trip into the wellbore is necessary between the forming of the wellbore and the lining of the wellbore.
Drilling with casing is especially useful in certain situations where an operator wants to drill and line a wellbore as quickly as possible to minimize the time the wellbore remains unlined and subject to collapse or the effects of pressure anomalies. For example, when forming a subsea wellbore, the initial length of wellbore extending downwards from the ocean floor is subject to cave in or collapse due to soft formations at the ocean floor. Additionally, sections of a wellbore that intersect areas of high pressure can lead to damage of the wellbore between the time the wellbore is formed and when it is lined. An area of exceptionally low pressure will drain expensive drilling fluid from the wellbore between the time it is intersected and when the wellbore is lined. In each of these instances, the problems can be eliminated or their effects reduced by drilling with casing.
While one-pass drilling offers obvious advantages over a conventional completion operation, there are some additional problems using the technology to form a subsea well because of the sealing requirements necessary in a high-pressure environment at the ocean floor. Generally, the subsea wellhead comprises a casing hanger with a locking mechanism and a landing shoulder while the string of casing includes a sealing assembly and a casing mandrel for landing in the wellhead. Typically, the subsea wellbore is drilled to a depth greater than the length of the casing, thereby allowing the casing string and the casing mandrel to easily seat in the wellhead as the string of casing is inserted into the subsea wellbore. However, in a one-pass completion operation, the casing is rotated as the wellbore is formed and landing the casing mandrel in the wellhead would necessarily involve rotating the sealing surfaces of the casing mandrel and the sealing surfaces of the wellhead. Additionally, in one-pass completion an obstruction may be encountered while drilling with casing, whereby the casing hanger may not be able to move axially downward far enough to land in the subsea wellhead, resulting in the inability to seal the subsea wellhead.
A need therefore exists for a method of drilling with casing that facilitates the landing of a casing hanger in a subsea wellhead. There is a further need for a method that prevents damage to the seal assembly as the casing mandrel seats in the casing hanger. There is yet a further need for a method for landing a casing hanger in a subsea wellhead after an obstruction is encountered during the drilling operation.